Raman scattering was discovered by C. V. Raman in the 1920's when he observed that visible monochromatic light is colorshifted during light scattering by compounds dissolved in solution. Light incident on a molecule must be scattered from induced electronic dipoles for Raman scattering to occur.
A related phenomenon to Raman scattering is the absorptive scattering of infrared light from molecules. In this case, infrared light is absorbed as a molecule makes transition between various rotational or vibrational energy level. This results in reduced intensity at the energies corresponding to those transitions.
For Raman scattering, monochromatic light is scattered by an electronic transition which loses energy during the scattering, the energy being that of a transition from one rotational or vibrational level to another. The corresponding wavelength shifts in the light can be measured and provide a spectral signature for chemical identification similar to infrared light spectra. Such spectral signature is also known as a "fingerprint" or identification spectrum of the chemical material.
Knowledge of the relative intensities and positions of these scattered light signals are obtained from a monochromator that disperses visible light. An adequate spectrum of the target molecule can be obtained when a large enough portion of the spectrum is measured. Thus, the monochromator must be capable of analyzing light throughout the visible range and near the infrared region.
One of the major advantages of compound identification by Raman scattering is that even slightly different molecules will display unique Raman spectra. The accuracy of the Raman spectrum measurements is basically determined by the sensitivity of the light detector, the dispersion capability and other optical devices, and the ability of the compound to scatter Raman light.
If a compound is a good Raman scatterer, then the optics can be arranged to make high resolution measurements thereby increasing the ability to differentiate compounds. If the sample is not a good Raman scatterer, then appropriate alterations must be made in the experimental apparatus so that weak Raman light can be detected. Unfortunately, resolution is usually compromised when it is necessary to detect low light intensities and resolve the spectra of different compounds. Regardless, the Raman spectra will not change as long as the molecular structure is not altered or if nonlinear processes are not induced by large laser light intensities.
Various known reliably reproducible methods of material analysis include chromatography and mass spectrometry. However, these known techniques involve destruction of the specimen being analyzed. The primary advantages that Raman scattering detection processes have over conventional detection methods is that they are rapid and nondestructive, yield a "fingerprint" of the compound in question (i.e. the identification spectrum) with high sensitivity, and are applicable for measurements in or out of solution. As noted, gas chromatography, high performance liquid chromatography (HPLC) and mass spectrometry are destructive and relatively slow compared to Raman spectroscopy. Furthermore, infrared adsorption spectroscopy is not simple to perform in aqueous solutions since water strongly absorbs infrared light across a broad wavelength range.
The primary disadvantage of Raman scattering detection techniques is that Raman scattering is a weak process. Raman spectroscopy has low sensitivity requiring the use of powerful, costly laser sources for excitation. Lengthy experimental procedures and/or rather large quantities (milligrams) of the material being analyzed are sometimes required to obtain a good signal. The cost of the equipment is comparable to that of conventional detection methods.
The apparatus commonly used in Raman scattering includes a visible light laser, an optical spectrometer, and various optical devices such as lenses, light filters and mirrors. For Raman scattering measurements on bulk chemicals, the material is collected in some type of transparent container and laser light is allowed to strike the contents. Solid material may also be analyzed without containment in a sample vessel. The light scattered from the material is then collected by the lenses and other optical devices and focused into the entrance port of the spectrometer.
Measurement of the intensities and wavelengths of the scattered light is performed by the spectrometer. The empirical data is then transmitted to the data storage device which is usually a computer. The operator may then store the data or obtain a hard copy of the results obtained by the spectrometer.
Although this type of phenomenon has been known for years, to date, a good commercially useful Raman scattering spectroscopy system capable of achieving consistently reliable results still remains unavailable. In 1974, surface-enhanced Raman spectroscopy (SERS) was first discovered using an electrochemical cell having a solution with buffer agents. This particular type Raman spectroscopy system detects scattered monochromatic light from an adsorbate specimen constituting a target for a light beam. Compounds placed at the surface of a microbase may be analyzed and identified based upon their characteristic Raman spectrum. While compounds in solution will be adjacent the SERS-active surface, dry techniques have also been developed to coat the compounds being analyzed directly onto the SERS-active surface.
In 1978, the improvement referred to as surface-enhanced Raman spectrometry was explained as a particular form of the general field of surface analysis spectroscopy. The Raman scattering intensity for adsorbates on or near a special rough metal surface have been enhanced by factors of 10.sup.3 to 10.sup.6 times. Such known enhancements have been achieved at silver, copper and gold metal surfaces under both solution and dry vacuum conditions.
SERS studies have involved both the use of rigid and flexible substrates. Microscopically roughened surfaces have been covered with particles of metal such as silver or the like and used as supports for adsorbates in the SERS procedure. However, one of the recognized problems related to SERS is the lack of a practical substrate material that can be easily prepared and provide SERS data with sufficient reproducibility and accuracy for effecting commercial analytical purposes. U.S. Pat. No. 4,674,878 teaches the use of a flexible substrate and is incorporated in its entirety herein by reference.
Several known techniques are used for producing rigid microbase substrates. Such techniques include electrochemical roughening of electrode surfaces, a lithographic process and the prolate post or etched island method. Various types of microbodies including roughness-imparting microspheres, submicronsized beads and nonspherical particles such as submicron needles have been used to produce results with the SERS technique.
More specifically, substrates including SERS-active surfaces having microneedles with various shapes and sizes disposed thereon have been used for SERS analysis of materials. Although the possibility of developing a portable, SERS system has been contemplated, no known process presently exists to commercially produce microbases having consistently reliable SERS results to make such a portable SERS system feasible.
In a known method of producing a microbase, a 200 nm (nanometer) deposited film thickness of calcium fluoride provided a first roughened layer onto a glass substrate. Next, an 80 nm deposited thickness of silver metal was produced at normal incidence to form a good conducting layer. A final silver evaporation then took place at a grazing incidence and at a rate of 2 nm per second with the length of the submicron needles being almost equal to the total evaporation or deposited thickness. See article entitled "Optical Properties of Submicrometer-size Silver Needles" published May 15, 1988 in Volume 7, No. 14 of the Journal for the American Physical Society."
All deposited or evaporation film thicknesses are measured in a well known manner with a quartz crystal thickness monitor. All evaporations took place in a cryopumped electron beam evaporator at a vacuum pressure of 1.times.10.sup.-6 torr. The average deposited thickness of the silver was reported at 210 nm and resulted in needles of approximately 200 nm or 2000 angstroms in length and 30 nm or 300 angstroms in width. Duplications of this reported experimental process failed to reproduce the results as reported in the May 15, 1988 article.
In another reported process for producing several microbases, a layer of calcium fluoride having a deposited thickness of 210 nm was first placed on a rigid substrate followed by the deposition of various evaporation or deposited thicknesses of 100, 150 and 200 nm of metals at deposition rates of 1 to 1.5 nm per second. The spaced distance between the evaporant crucible holding the metal being evaporated and the sample substrate on which the metal was being deposited was 30 cm. The sample was positioned at an incidence angle of 88.degree. with respect to the evaporant crucible. A gold overlay of about 7 nm was disposed over the microneedles to prevent severe charging problems. See article in the Journal of the Optical Society of America, Volume 5, page 2552, December 1988 entitled "Surface Electromagnetic Modes in Prolate Spheroids of Gold, Aluminum, and Copper".
A further prior art technique is disclosed in a paper entitled "Optical and Microstructural Properties of Obliquely Evaporated Silver Films on Rough and Smooth Substrates." Various optical absorbance spectra are disclosed for obliquely evaporated silver films on microscope slides with either a 50 nm deposited layer of calcium fluoride or a 300 nm deposited layer of calcium fluoride. All evaporation or deposited thicknesses of the silver were monitored at 200 nm and the respective substrate slides placed at incidence angles of 89.3.degree., 89.degree., and 87.4.degree.. No relationship is disclosed regarding the usefulness of these substrates in a SERS system. However, duplication of the reported process produced a target microbase which did not achieve commercially viable SERS data.
In another reported procedure, a 210 nm deposited layer of calcium fluoride was first placed on a microscope slide followed by a second contiguous 65 nm deposited layer of magnesium fluoride. Three silver evaporation or deposited thicknesses were tested on the two-layered surface at 100 nm, 200 nm, and 300 nm of evaporation as determined by the standard quartz monitor. All silver evaporations took place at an incidence angle of 88.degree..
Although the deposited or evaporation thickness as determined by the quartz thickness monitor reached 300 nm in these prior art processes, the length of the resultant needles attained a maximum of 200 nm or 2000 angstroms in length and 300 angstroms in width. Furthermore, attempts to consistently reproduce the reported microbase structures have been unsuccessful. In each of these prior art processes, the resultant microbases did not produce consistently reliable SERS results capable of achieving commercial reproducibility.